Watch Your Mouth: When to Taste—or Toss—Your Leftovers

If you are, though, it’s a bit late to stock up on non-perishables. Now that the latest summer storm has petered out in the North Atlantic, something else may be brewing inside your refrigerator. Is your frozen chicken still safe? When the electricity went, should you have finished off the cheese before cleaning out the leftover fried rice? Is milk drinkable after its Sell By date?

Whether you’re prepping for Hurricane Katia, this weekend’s barbecue, or the impending apocalypse, less-than-chilled food presents a culinary dilemma: Toss it out prematurely and waste good food, or hold on to it too long and risk getting sick. Public officials generally err on the side of caution. Listen to the familiar principle the Red Cross and the Centers for Disease Control and Prevention have been parroting for the past week: When in doubt, throw it out.

After all, microbes are everywhere. Some are essential to human life, others can cause food to spoil, and a few can kill you. Humans have attempted to combat the effects of bad microbes with fermentation, pasteurization, irradiation, and refrigeration. But the only surefire way to eradicate all microbial life is to pressure-cook your food for hours, then eat it immediately.As Harold McGee writes in Keys to Good Cooking, “Such food would also be pleasure free.” Anyone who's sampled food from their local hospital's cook-chill operation is acutely aware of the challenges of making safe food palatable.

So how does an eater strike a happy medium between taste and poison? Aside from downing sterile mush—or irradiated foil packets of MREs (known to soldiers as Mr. Es)—the most ubiquitous, and least expensive, technology for staving off contamination is refrigeration. An electric fridge retards bacterial growth, but it’s not foolproof, as food scientist O. Peter Snyder writes in “Food Safety Hazards and Controls for the Home Food Preparer" [PDF]. “Spoilage bacteria begin to multiply at 23°F,” he writes. So once raw food starts to thaw, “it actually begins to spoil.”

In the case of some deli meats, added preservatives can slow the growth of pathogenic bacteria. But a longer life isn't always worth living. “These products may develop a slimy surface after being kept for a period of time,” Snyder writes. “This slime is due to the growth of spoilage bacteria. However, the meat is probably safe to consume even though its quality characteristics make it objectionable.” Even if its microbial ecology might not cause physical harm, slimy bologna could still turn your stomach. The same goes for pasteurized milk: Heat kills pathogens, but milk left out over time still goes sour and curdles.

So when disaster strikes, it's best to have a consumption strategy on hand. When the electricity goes out and your fridge warms up, the U.S. Department of Agriculture's Food Safety and Inspection Service (FSIS) recommends eating thawed meats, soft cheeses, and eggs within four hours of an outage. This four-hour rule also applies to milk and fruit that’s been cut up. Four hours is hardly enough time to make that stuff appear slimy or smell sour. But even in food that looks and smell fine, pathogenic bacteria begin to proliferate at temperatures above 40° Fahrenheit.

The four-hour rule comes from the 698-page 2009 federal Food Code and is based on a worst-case scenario calculation from the USDA’s Pathogen Modeling Program. Scientists found that in the lab, Listeria, a pathogenic bacteria found in soil, grew exponentially inside a broth held at room temperature within four hours. While actual foods that can have Listeria—hot dogs, raw milk, queso blanco, and potatoes—don’t reach 75° Fahrenheit in an unplugged fridge, tossing them at the four hour thresholdpractically guarantees thatListeria and other pathogens will be kept at bay.

“The four-hour rule is clear and unambiguous,” Don Schaffner, a food scientist at Rutgers, told me, “but the reality is more complicated.” Bacteria grows at a certain temperature threshold, not after an exact period of time; the time limit is just a rough approximation for those of us who don't have thermometers at the ready. And that four-hour window also fails to take salt, nitrates, oracidity into account, all of which inhibit microbial life(think beef jerky, ketchup, and pickles). Not to mention how many times you’ve procrasti-snacked in front of the open fridge door,letting heat sneak in one string cheese at a time. As a rule, Schaffner says, “The warmer a food is, the longer the time, the higher the risk.”

The dynamic shifts when things get really warm—you fire up the grill and start cooking. Even old, unrefrigerated burger meat or slimy, wet onions—both teeming with millions of microorganisms per gram—can be safe if heated to a certain temperature. Where refrigeration is less common, herbs, spices, and cultural acceptance can sometimes mask off-flavors or odors associated with spoilage. As the FSIS notes, “Most people would not choose to eat spoiled food. However, if they did, they probably would not get sick.” And Nathan Myhrvold, former Microsoft CTO, argues in Modernist Cuisine that the current temperature standards for killing specific quantities of pathogens are very conservative, and go “far beyond those supported by science.” We may be killing bad bacteria at 165° Fahrenheit, Myhrvold says, but in the process we’re overcooking chicken, too.

But the better-safe-than-sorry approach stems from the fact that eaterscannot sniff outpathogens. It’s impossible to taste harmful Staph in meat or Baccilus in rice, and cooking will kill the bacteria but not the heat-stable toxins these bacteria produce—stuff that could cause nausea, vomiting, cramps, and even death. As Harold McGee wrote in The New York Times last week, keeping rice around at room temperature has had fatal consequences. “Cooking reduces risk,” Schaffner told me, “but it doesn’t necessary make an unsafe food safe under all circumstances.” It’s still safe to say: When in doubt, throw it out.

Of all the threats we face—earthquakes, hurricanes,or world-ending existential events—home cooking may not rank all that high. Of the dozen Americans killed and 200,000 sickened by food every day, an estimated 70 percent of food-borne bacteria come from farms and restaurants outside the home, as Mike Batz told me earlier this year. Unlike natural disasters, food poisoning is preventable. What we need is a thermometer in the kitchen and a more watchful governmental eye.